1. Field of Invention
The present invention relates to a projection screen technology, and more particularly, to a method of producing a transmissive screen used as a display screen of a projection television or a microfilm reader, for example. The invention also relates to a transmissive screen produced by this method.
2. Description of Related Art
In recent years, rear projectors that use a liquid crystal light valve or a CRT as a large screen display have been the focus of attention. The display displays an image by forming an image on a transmissive screen using image light from an image projecting portion. This type of transmissive screen is bright when an observer observes the image and has predetermined very small lens members formed thereon so as to increase the viewing angle.
As shown in FIG. 11, with regard to light distribution property of such a bright screen that has a wide viewing angle, a viewing angle 1101, which is wider in the horizontal direction than in the vertical direction, is preferred. This is because the viewing angle of a human being is wider in the horizontal direction than in the vertical direction. When the light distribution is made to be equal in the vertical and horizontal directions, light is also distributed in the vertical direction which is not actually necessary with regard to the viewing angle of a human being, so that the brightness as a whole is reduced.
Representative examples of the structures of the transmissive screen include the following:
{circle around (1)} A lenticular sheet includes a lens portion that is formed by providing convex cylindrical lenses (semi-circular cylindrical convex lenses) side by side. As shown in FIG. 9(a), in general, the lenticular sheet has a structure that is formed by forming both surfaces of the sheet into convex cylindrical lens surfaces 901, forming protrusions at boundary portions between the respective cylindrical lenses at one of the surfaces of the sheet (the surface from which image light 201 exits), and forming a light-shielding layer (a black stripe having a light-absorption property) 902 on the top portion of each of the protrusions.
The lenticular sheet is obtained by performing a press-molding operation on a transparent thermoplastic resin sheet or by molding both surfaces of the resin sheet at the same time that molten extrusion is performed.
{circle around (2)} A planar lens has very small transparent balls arranged two-dimensionally (disclosed in, for example, U.S. Pat. Nos. 2,378,252 and 3,552,822, and Japanese Utility Model Registration Gazette No. 2513508). As shown in FIG. 10(a), in the planar lens, each of the very small transparent balls 1002 has approximately 50% of its diameter embedded in, and held by, a light-incident side transparent layer 1001, and the remaining approximately 50% embedded in a light-exiting side light absorption layer 1003.
The planar lens is obtained by forming a sheet that includes a transparent layer, very small transparent balls, and a light-absorption layer, and then bonding it to a transparent substrate 1004.
However, such related art transmissive screens are subject to the following problems.
In the lenticular lens, it is difficult to achieve a fine pitch when each of the above-described molding methods is performed on thermoplastic resin, so that when the lenticular lens is used as a screen of a rear projector, which in recent years has been used to provide increasingly higher definition, there is a problem in that deterioration of image quality occurs due to reduced resolution and production of moiré. In addition, a very small light diffusing material is typically mixed in the inside portion of the lenticular lens in order to increase the viewing angle in the vertical direction (a direction parallel to the lenticular lens, which is represented by reference numeral 903 in FIG. 9(b)) in which the lenticular lens does not have optical power. This gives rise to the problem that image quality is deteriorated because speckles are produced due to the interference of image light caused by the light-diffusing material. Further, both of the molding methods performed on the thermoplastic resin require large molding machines or dies having diagonals that are equal to or greater than 50 inches, which are of the same size as the screen of the rear projector, giving rise to the problem that production costs become very high.
On the other hand, in the planar lens having very small transparent balls that are arranged two-dimensionally, as shown in FIG. 10(b) in which the planar lens is viewed from an image light incident side, dead spaces, which do not pass image light, are formed between the individual very small balls 1002, so that the image light incident thereupon is not transmitted to the observer side. In addition, it is very difficult to completely perform a minute filling operation with respect to the very small balls, so that the dead spaces increase in size. Further, since the thin light absorption layer 1003 remains at the observer-side surfaces of the very small transparent balls, light is absorbed. Due to these three reasons, the problem arises that light transmittance of the transmissive screen is low.
Since the increase in the viewing angle by the very small balls is completely isotropic, light is also diffused in the vertical direction, in which the viewing angle does not normally need to be increased very much, to the same extent as in the horizontal direction. This gives rise to the problem of insufficient brightness when the image is viewed from the front.
In general, the planar lens is produced by the step of forming a sheet including a transparent layer, very small transparent balls, and a light absorption layer, and bonding the sheet to a transparent substrate. However, in the step of bonding the sheet to the transparent substrate, unevenness in the bonding occurs, so that the display of the image becomes non-uniform, and by insufficient adhesiveness between the sheet and the transparent substrate, interfacial multiple reflection occurs, thereby giving rise to the problem of reduced resolution.